1. Field of the Invention
The present invention relates to a sliding fluid level-detecting device which detects a remaining amount of a liquid fuel such as gasoline or gas oil placed in fuel tanks of various vehicles such as automobiles, motorcycles, buses, and trucks.
2. Related Art
A vehicle is equipped with a fuel gauge which informs its driver of a remaining amount of a liquid fuel in a fuel tank. The fuel gauge is mounted in the fuel tank and connected with a fluid level-detecting device which detects a fluid level of the liquid fuel, and a sliding fluid level-detecting device using a system of detecting displacement of a float floating on a liquid fuel has been widely used.
FIG. 1 is a schematic diagram illustrating one example of the sliding fluid level-detecting device. As shown in the figure, the sliding fluid level-detecting device is supported by a supporting board 2 on an inner wall 1a in the vicinity of opening of a fuel tank 1. The supporting board 2 is equipped with an insulation board 3 and, as shown in FIG. 2 in a magnified form, a plurality of electrodes 4, 4 are placed in an arc-shaped form. Furthermore, an arc-shaped resistive layer 5 extending in a band form is formed so as to cover the outer edge of the electrodes 4, 4 and thereby an electrode part 10 is constituted. Moreover, a terminal 7 provided at the end of a slider 6 comes into contact with the electrodes 4, 4.
In addition, the sliding fluid level-detecting device is equipped with a rod-like electrically insulating arm 8 and a float 9. The arm 8 is connected with a rotating base 6a of the slider 6 at the one end 8 and the arm 8 is capable of integrally rotating together with the slider 6, the rotating base 6a being a center. Moreover, the float 9 is connected with another end 8b of the arm and the float 9 floats on the liquid surface of a liquid fuel in the fuel tank 1 and is displaced in a vertical direction in the figure corresponding to the displacement of the fluid level.
In the sliding fluid level-detecting device constituted as mentioned above, when the float 9 is displaced corresponding to the fluid level of the liquid fuel, the arm 8 rotates, the rotating base 6a being a center. Following the movement, the slider 6 connected with the arm 8 also rotates, the rotating base 6a being a center. Following the rotation of the slider 6, a contact point 7 slides on the surface of the electrodes 4, 4 and, at that time, the remaining amount of the liquid fuel is detected based on the resistance value between the terminal 7 and an endmost part 5b of the resistive layer 5.
In such a sliding fluid level-detecting device, as a material for forming the contact point 7 and the electrode 4, a silver-palladium alloy or a silver-nickel alloy has been hitherto employed. However, since the electrode part 10 has been immersed in the liquid fuel in the fuel tank 1, there has arisen a problem that sulfur matter contained in the liquid fuel reacts with the silver-palladium alloy or silver-nickel alloy to form silver sulfide, an insulating material, on the surface of the contact point 7 and electrode 4, the formation invites an insufficient contact between the contact point 7 and the electrode 4, and thereby a detection accuracy decreases with time.
Thus, in order to prevent the formation of silver sulfide, it has been proposed to form the contact point 7 and the electrode 4 out of a glass-incorporated silver-palladium alloy where glass is mixed with a silver alloy such as a silver-palladium alloy or a silver-nickel alloy (see, e.g., Patent Documents JP-A-2002-202179, JP-A-2003-287455 and JP-A-2003-287456).
In conventional glass-incorporated silver alloys, the admixed amount of glass is in the range of 10 to 15 parts by weight per 100 parts by weight of a silver alloy and it is observed that the glass-incorporated silver alloys are effective for highly purified gasoline and gas oil having a sulfur concentration of 10 to several dozen ppm. However, gasoline and gas oil having a sulfur concentration of near to 100 ppm have been used particularly abroad and thus there is a possibility that a sulfuration-preventing effect becomes insufficient in the case of conventional glass-incorporated silver alloys.